What Controls the Sequestration of Phytogenic Carbon in the Ocean?

Abstract

A major fraction of phytogenic organic carbon, produced in the surface ocean, is processed by the heterotrophic biota in the water column, whereas only a minor fraction is buried in marine sediments. The depth distribution of organic carbon mineralisation in the water column determines the time scales for carbon dioxide withdrawal from the atmosphere. The transfer of organic carbon from the surface to the deep ocean through gravitational settling and downward advection are therefore key processes in determining the efficiency of the biological pump. Sediment trap studies indicate the downward flux of particulate organic carbon to decrease exponentially with increasing water depth. The vertical export flux from the euphotic zone correlates positively with primary production up to productivities of~200 g C m-2 y-1 and attains a constant value at higher production rates. The amount and quality of organic carbon exported from the surface mixed layer strongly depends on the food web structure in the upper ocean. Some of the current perspectives on the role of grazing by micro- and mesozooplankton on retention versus export of organic carbon in the mixed layer are presented. The transformation through solubilization, respiration and fragmentation of aggregates and fecal pellets by particle associated food webs diminishes the downward flux of particulate organic carbon with increasing water depth. Calculated and measured metabolic activities of these food webs suggest turn-over times of the aggregate organic carbon in the order of weeks. The hydrodynamics of sinking particles appears crucial for the mass exchange between the particle and its surrounding water and regulates the activity of the associated community. In addition, while sinking through the water column the particle associated communities are exposed to changes in physico-chemical conditions such as temperature and oxygen concentration which, by controlling the activity of the community, may regulate the downward flux of particulate organic carbon in deeper waters. Our current understanding of biological processes, especially in the upper water column, is insufficient to predict the composition and downward fluxes of organic matter.